The safety of vehicles and highways has been steadily improving over the past few decades to the point that, on an individual basis, fatal accidents are relative rare. In fact, fatal accidents are now less than 1.7 per 100,000,000 miles traveled. This means that further improvements in vehicle design and infrastructure will produce progressively smaller reductions in rates at increasing costs. Put another way, the easy problems have been solved and it is becoming increasingly difficult to realize further improvements.
A recent study by Dupont Health and Safety showed that the numbers of fatalities and lost work days were so small that it was virtually impossible to reduce them further. The recorded injuries showed random casualty, and it was determined that it would be ineffective to work on reducing such random occurrences to any significant degree. The solution, they found, is in reducing near accidents, and particularly in understanding and reducing risk/behavior.
The same principle holds for highway accidents. Namely, that serious injuries and fatalities are usually the probablistic result of repeated patterns of risky behavior. If risky behavior can be reduced, a reduction in all types of accidents may be realized overall.